Carbon nanostructures are attracting attention as core materials in nanotechnology. In the present disclosure, the term “carbon nanostructure” is used to refer to a nano-sized material composed of carbon atoms. Examples include coil-shaped carbon nanocoils, tube-shaped carbon nanotubes (also referred to below as “CNTs”), carbon nanotwists in which CNTs are twisted, bead-supporting CNTs in which beads are formed on CNTs, carbon nanobrushes composed of numerous stood-up CNTs, spherical shell-shaped fullerenes, graphene, and thin films of diamond-like carbon. Such carbon nanostructures are related in terms that a nanostructure containing sp2 hybridized carbon is deposited on the surface of a metal (catalyst) by chemical vapor deposition and there are many analogies in the production methods of such carbon nanostructures.
A currently known method involves supplying a feedstock gas to a catalyst and growing a carbon nanostructure by chemical vapor deposition (also referred to below as “CVD”). In the method described above, a feedstock gas including a carbon compound is supplied to fine metal particles serving as a catalyst at a high ambient temperature of approximately 500° C. to 1,000° C. Various carbon nanostructures can be produced through the above-described method by making various alterations to the catalyst type, catalyst positioning, feedstock gas type, reaction conditions, and so forth.
For example, PTL 1 discloses a method in which methane (CH4) or ethylene (C2H4) is used as a feedstock gas and in which CNTs are produced through CVD in a state in which the catalyst is in contact with an alkyne-containing gas. PTL 2 discloses a method in which a hydrocarbon gas such as methane, ethylene, or acetylene (C2H2) is used as a feedstock gas and in which CNTs are produced through CVD by blowing the feedstock gas onto the catalyst.
CNT production techniques using CVD are advantageous in terms that both single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) can be produced and that use of a substrate with a catalyst supported thereon enables production of numerous CNTs that are oriented perpendicularly to the substrate surface. One method suitable for CNT mass production that has attracted much interest since development thereof is a super growth method in which a feedstock gas is brought into contact with a catalyst together with a catalyst activating material such as water.
It should be noted that although the chemical reaction mechanism by which CNTs are synthesized through CVD is not yet fully understood, a number of research studies have reported that acetylene (alkyne) is an effective molecule in CNT synthesis (in other words, a molecule that actually serves as a CNT precursor). For example, NPL 1 asserts that acetylene serves directly as a CNT precursor based on the results of an experiment in which plasma/thermal CVD was carried out using methane as a carbon feedstock. NPL 2 proposes that a polymerization reaction of ethylene and an alkyne, such as acetylene, is an efficient CNT synthesis mechanism based on the results of an experiment in which thermal CVD was carried out using ethylene as a carbon feedstock. NPL 3 asserts that acetylene is an effective CNT precursor based on the results of an experiment in which thermal CVD was carried out using ethanol as a carbon feedstock.
It can be inferred from the preceding studies described above that it is highly probable that feedstock gases conventionally known to be suitable for CNT synthesis (methane, ethylene, and ethanol) are not precursors that contribute directly to CNT synthesis and that it is also highly probable that acetylene (alkyne) serves as a CNT precursor.